The bulk physical properties of biological membranes are controlled by the structure of their component phospholipids. The fatty acid composition and positional distribution of acyl moieties in these phospholipids is rigorously controlled. The biochemical mechanisms responsible for this specificity are poorly understood, but the bulk of the available evidence points to the sn-glycerol-3-phosphate acyltransferase system as the primary site responsible for the control of phospholipid structure. The basic goal of this research is to construct a model for the regulation of membrane phospholipid structure. The kinetics and specificity of the acyltransferase will be examined using the physiological acyl donor, acyl-acyl carrier protein. A hypothesis to be tested is that the positional asymmetry of fatty acids in membrane phospholipids is not due simply to the acyltransferase having an absolute specificity for a particular fatty acid, but that ancillary enzymatic activities are also present that function as editing or proofreading steps in the overall sequence. Since the specificity of the acyltransferase is proposed to be regulated by a kinetic mechanism, the size and composition of the acyl-acyl carrier protein pool available to the acyltransferase in vivo will be determined to relate the results obtained in vitro to the physiological situation.